The present invention relates to the measurement of liquid flow rates.
Semiconductor manufacturers, the health industry, the food industry and others require accurate measurement of low liquid flow rates. Low liquid flow rates tend to break up into a series of droplets of small volume. As the desired accuracy approaches the droplet volume, measurements become unreliable.
U.S. Pat. No. 4,869,722 describes a device that tries to address this problem by monitoring the droplets released from the tip of a drop funnel 13 onto a liquid at the top of a reaction cell 16. The reaction cell 16 senses the impact of the droplet, but the user cannot readily control the droplet size or flow rate and the measurements are susceptible to environmental change and minor physical perturbations. The droplet size cannot change without altering the funnel opening diameter, and the liquid flow rate changes with variations in liquid supply height. Sampling of a droplet is also inaccurate if the droplet forms at the funnel opening due to surface tension or falls in mid-air before impact between measurement intervals. This method is also unsuitable for measuring higher liquid flow rates not forming droplets, thus limiting its applications.
U.S. Pat. No. 4,244,218 and U.S. Pat. No. 6,026,683 use the buoyancy principle to measure liquid level and flow rate. Those methods are not suitable for very low flow rate measurements where error tolerances exceed the droplet or sub-droplet range. Floatation devices have delicate mechanical parts prone to calibration errors and physical damage, adding to manufacturing and maintenance costs.